The present invention relates to a wavelength-selective polarization conversion element which converts non-polarized light into polarized light with a direction depending on wavelength regions (colors) and an image projection apparatus such as a liquid crystal projector including the wavelength-selective polarization conversion element.
Each of Japanese Patent Laid-Open No. 2001-154152 and U.S. Pat. No. 6,742,897 has disclosed an image projection apparatus which performs color separation and color combination with a polarization beam splitter. In the apparatus, non-polarized light emitted from a light source is split into a plurality of luminous fluxes by a lens array, and secondary images of the light source are formed by the respective luminous fluxes and are caused to overlap with each other on an image-forming element such as a liquid crystal panel by a condenser lens to illuminate the image-forming element at substantially uniform brightness.
The split luminous fluxes that emerged from the lens array enter a plurality of polarization conversion cells provided in a polarization conversion element such that the cells are associated with lens cells of the lens array. Each of the polarization conversion cells has a polarization beam splitting film, a half-wave plate and a reflecting surface. Non-polarized light entering each polarization conversion cell is split into P-polarized light and S-polarized light by the polarization beam splitting film. The P-polarized light is transmitted through the polarization beam splitting film. The polarization direction thereof is rotated by 90 degrees by the half-wave plate, and then the resulting S-polarized light emerges therefrom. On the other hand, the S-polarized light is reflected by the polarization beam splitting film, reflected by the reflecting surface, and then emerges without any change.
The S-polarized light with only one polarization direction emerges from the polarization conversion element and enters the condenser lens. The S-polarized light then emerges from the condenser lens and is split into light in first and second wavelength regions and light in a third wavelength region by a dichroic element. At this point, the light in the first wavelength region and the light in the second wavelength region travel along the same optical path and have the same polarization direction. Then, the light in the first and second wavelength regions is passed through a wavelength (color)-selective phase plate in order to direct the light in the first wavelength region and light in the second wavelength region to a first image-forming element and a second image-forming element, respectively, with a polarization beam splitter. This can split the light in the first and second wavelength regions into light components polarized in different directions. The wavelength-selective phase plate is formed by laminating plural stretched films and has a function of converting only one of the light in the first wavelength region and the light in the second wavelength region into light with a polarization direction orthogonal to the original one and transmitting the light in the other wavelength region without changing its polarization direction.
The image projection apparatus which achieves color separation with the polarization beam splitter as described above typically includes the polarization conversion element placed near the lens array and the wavelength-selective phase plate disposed near the polarization beam splitter.
A particular optical element disclosed in each of U.S. Pat. No. 6,742,897 and Japanese Patent Laid-Open No. H11(1999)-153774. U.S. Pat. No. 6,742,897 has disclosed an optical element formed of a first dichroic layer which transmits light in a predetermined wavelength region and reflects light in other wavelength regions, a phase layer which rotates the plane of polarization of the light transmitted through the first dichroic layer by 90 degrees, and a total reflection layer which totally reflects the light from the phase layer.
A conventional polarization beam splitter is intended to transmit P-polarized light and reflect S-polarized light over the entire used wavelength region. A polarization beam splitter which reflects P-polarized light and transmits S-polarized light has been reported in “Li Li and J. A. Dobrowolski, Appl. Opt., vol. 39, p. 2754, 2000”. Any polarization beam splitter has a function of achieving polarization split over the entire used wavelength region. No report has been made on a polarization beam splitter which transmits S-polarized light and reflects P-polarized light in a certain wavelength region, and reflects S-polarized light and transmits P-polarized light in another wavelength region.
In the dichroic filter, the transmission wavelength region of P-polarized light is widened and the transmission wavelength region of S-polarized light is narrowed when light is incident obliquely thereon, so that polarization split is realized in a particular wavelength region. However, P-polarized light and S-polarized light are transmitted and reflected, respectively, at all times. P-polarized light is not reflected or S-polarized light is not transmitted. The polarization split characteristic is not reversed depending on the wavelength region.
Japanese Patent Laid-Open No. 11(1999)-153774 has disclosed a polarization beam splitter which has wavelength selectivity and performs polarization of light, analysis of light, color separation and color combination. It has disclosed a color separation/combination means which includes the polarization beam splitter having a function of reflecting P-polarized light and transmitting S-polarized light in a wavelength region of blue and transmitting P-polarized light and reflecting S-polarized light in wavelength regions of green and red. However, only the function of the polarization beam splitter has been described, and no disclosure has been made of how to embody it.
When the wavelength-selective phase plate is used in addition to the polarization conversion element as disclosed in Japanese Patent Laid-Open No. 2001-154152, however, the number of optical components of the image projection apparatus is increased. It is also necessary to provide a member for positioning and supporting the wavelength-selective phase plate and a member for cooling the wavelength-selective phase plate formed of the multilayer film, thereby complicating the structure.